The present invention relates generally to the field of the processing of the signal from a mobile, airborne radar. The disclosure relates to detection of a low-energy target that may be concealed by high-energy reflectors.
Conventional radars use radial velocity distance cartography to separate the echoes from all of the targets in terms of distance and velocity. This amounts to positioning the targets in a radial velocity distance space discretized in cells, in a manner known by those skilled in the art, as shown in FIG. 15b. 
In that figure, one pylon PYL1 is situated at a distance d1 from the radar system; two pylons PYL2 and PYL3 are situated at a same distance d2, but seen from different angles, as shown in FIG. 8.
Using conventional distance processing, the radar system obtains a vector, called the distance profile, illustrated in FIG. 15a. In this profile, the pylon PYL1 will generate a local energy maximum in the distance cell corresponding to the distance d1. Likewise, the pylons PYL2 and PYL3, which are located at the same distance d2 from the system, will generate a local energy maximum in the cell corresponding to said distance d2.
This distance processing makes it possible to distinguish the pylon PYL1 from the pylons PYL2 and PYL3 on the distance profile. It does not, however, make it possible to distinguish between pylons PYL2 and PYL3.
To resolve this problem, velocity processing is conventionally applied. The output of this processing over all of the observed distances constitutes a distance-radial velocity cartography of the environment of the system, and is diagrammatically illustrated in FIG. 15b. 
Owing to this velocity processing, it is now possible to distinguish between the two pylons PYL2 and PYL3. In fact, since they are seen from different angles, they have different radial velocities relative to the radar system, as illustrated by FIG. 9. Once this velocity difference is greater than the velocity resolution of the system, the two pylons PYL2 and PYL3 appear in different velocity cells.
Using conventional means, targets may be separated from their direct environment by comparing the relative energy levels on the distance velocity cartography, this direct environment being able to be made up only of noise, periodic reflectors, or extended reflectors. For example, and as shown in FIG. 16, a target 60 having energy greater than the clutter 50 can be separated from the latter.
However, a mobile target C situated in the direct environment of the pylon PYL2 (see FIG. 10), and the energy of which is at the same level as that of PYL2, is concealed by the pylon and cannot be detected.
In certain flight and observation configurations, the strong echoes can therefore lastingly conceal targets and prevent them from being detected. This concealment may also be caused by folding phenomena.
One solution may be to try to eliminate the aliasing phenomena by varying the repetitive frequency of the radar waves. This method can be used when the target is found by velocity folding in the zone of the clutter, and consists of moving the aliasing velocity so that the target is located outside the zone of the clutter for one of the repetitive frequencies. However, there is may be no repetitive frequency that makes it possible to observe the target outside the clutter zone. This solution may not be fully satisfactory.
Another solution may be to use algorithms of the STAP (Space Time Adaptive Processing) type and antenna arrays to reduce strong targets such as ground clutter. The STAP algorithms use an array of horizontal receiving antennas to exploit the angle of arrival of the targets and to discriminate along the azimuth angle in the Doppler plane, the radial velocity of an echo of the clutter being connected to its angle of arrival. However, using arrays of antennas and applying STAP algorithms imposes major constraints on the sizing of the system and increases the processing complexity.
Another solution may be to separate targets using the polarimetric properties of the targets and the clutter. This method requires at least two transmitting antennas and at least two receiving antennas, which makes its implementation more complex.
Also a method was presented by Wang et al. in the document “Maneuvering target detection in over-the-horizon radar using adaptive clutter rejection and adaptive chirplet transform,” and published on Nov. 4, 2003, in the review IEE Proc. Radar Navig, Vol 150, No 4. This method makes it possible to detect moving targets on sea clutter by rejecting clutter through projection, then performing an iterative two-dimensional velocity processing through chirplet transform on the signal. However, this method is only applicable to a fixed radar. It may not be effective to reject ground clutter.